The invention relates to gain control or multiplier circuits of the type having an ac signal input and an amplified ac signal output wherein the output signal waveform is a current controlled amplified version of the input waveform. In particular, this invention is applicable to circuits employing antilog processing of ac signals.
The use of balanced semiconductor pairs is well known in the art and is widely practiced in the field of lower performance integrated circuit gain elements. These circuits generally suffer from poor control voltage rejection due to mismatches in the base-emitter voltages of differential pairs of semiconductors of the circuit and are limited in dynamic range due to the requirement that the input differential pair stage must be operated at a quiescent current in excess of the greatest input signal expected. Currently, the art deals with these limitations, though inadequately, by (a) integrating the transistors of a differential pair onto a common substrate to assure good matching at the expense of dynamic range because of the small geometry of the transistors used, or (b) using selected large geometry discrete transistors and trimming out the voltage offsets.
The invention disclosed and claimed herein is directed to current controlled amplifier systems or multiplier systems which overcome the aforementioned disadvantages. In particular, this invention utilizes a low pass filtered feedback means around the output stage of what may be an otherwise conventional dual differential pair multiplier so as automatically to compensate for mismatched base-emitter voltage offsets of the semiconductors forming the differential pair associated with the output amplifier stage.
In one aspect, this invention concerns, in a current-controlled amplifier circuit, the combination of output amplifier means having an input conductor and having an output conductor for converting a signal at the input conductor of the output amplifier means to a current amplified signal, feedback means connected between the output and input conductors of the output amplifier means and including a differential pair of semiconductors having a control current output, for presenting a signal at the input conductor of the output amplifier means which is the product of an input signal to the amplifier circuit and the ratio of a reference current to the control current and for substantially eliminating dc input at the input conductor of the output amplifier means and setting the quiescent currents of the semiconductors of the differential pair substantially equal.
In particular, in accord with the preceding object, the invention involves feedback means which includes a low pass filter circuit connected as an input to the differential pair. Further, the feedback circuit also includes a current mirror circuit associated with the differential pair. Still further, the output amplifier means is provided with a high pass filter feedback circuit.
More specifically, this invention concerns a current-controlled amplifier circuit comprising the combination of input amplifier means having an input conductor for receiving an input signal of ac waveform and having an output conductor, feedback means connected between the output and input conductors of the input amplifier means and including a first differential pair of semiconductors having a reference current output port, for creating an output waveform at the output conductor of the input amplifier means which is the product of the input signal and the reference current, output amplifier means having an input conductor and having an output conductor for converting a signal at the input conductor of the output amplifier means to a current amplified version of the input signal waveform, second feedback means connected between the output and input conductors of the output amplifier means and including a differential pair of semiconductors having a control current output, for multiplying the output signal of the input amplifier means by the control current to present a signal at the input conductor of the output amplifier means which is the product of the input signal at the input conductor of the input amplifier means and the ratio of the reference current to the control current and for substantially eliminating dc input at the input conductor of the output amplifier means and setting the quiescent currents of the semiconductors of the second differential pair substantially equal.
Still more specifically, this invention is concerned with a current-controlled amplifier circuit comprising the combination of input amplifier means having an input conductor for receiving an input signal of ac waveform and having an output conductor, feedback means connected between the output and input conductors of the input amplifier means for creating an output waveform at the output conductor of the input amplifier means which is the product of the input signal and the reference current, the feedback means comprising parallel branches in which one branch comprises the current path of one semiconductor of a differential pair having an output port for the reference current and a fixed value input port connected to the one semiconductor, and the other branch comprising a current path of the other semiconductor of the differential pair in series with a current mirror, the other semiconductor having an input port connected with the output conductor of the input amplifier means, output amplifier means having an input conductor and having an output conductor, second feedback means connected between the output and input conductors of the output amplifier means for multiplying the output signal of the input amplifier means by the control current to present a signal at the input conductor of the output amplifier means which is the product of the input signal at the input conductor of the input amplifier means and the ratio of reference current to the control current, the second feed back means comprising parallel branches in which one branch comprises the current path of one semiconductor of a second differential semiconductor pair having a control current output port and an input port of one semiconductor of the second differential pair which is connected to the output conductor of the input amplifier means, and the other branch comprising the current path of the other semiconductor of the second differential pair in series with a current mirror and low pass filter means connected to an input port of the other semiconductor of the second differential pair for substantially eliminating dc input at the input conductor of the output amplifier means and for setting the quiescent currents of the semiconductors of the second differential pair substantially equal, and high pass feedback means across the output amplifier means for converting the signal at the input conductor of the output amplifier means to a current amplified version of the input signal waveform.
These and other objects of this invention will appear more clearly from the detailed description which follows.